Glaucoma, a leading cause of world blindness, is a group of disorders, characterized by irreversible damage to the optic nerve, or glaucomatous optic neuropathy, in which elevated intraocular pressure is the main causative risk factor. The only proven way to prevent the blindness of glaucoma is to control the intraocular pressure.
Clinical management of intraocular pressure can be achieved medically or surgically. Modern medical therapy for glaucoma began in the 1870s, with the introduction of pilocarpine and other cholinergic agonists. In the twentieth century, several compounds were introduced, such as alpha-2 agonists, beta-adrenergic antagonists, topical and systemic carbonic anhydrase inhibitors, and prostaglandins. However, glaucoma medication are not available or practical in many parts of the world, and are inadequate in many patients, despite availability. Hence the need for surgical methods to control the intraocular pressure.
Control of intraocular pressure can be achieved surgically by reducing the production of aqueous humor or by increasing its outflow. Operations to reduce production, referred to collectively as cyclodestructive surgery, destroy a portion of the ciliary body, the source of aqueous humor. Destructive elements over the years have included diathermy, cryotherapy and, most recently, laser energy. While these operations are effective in lowering the intraocular pressure, and are beneficial in desperate cases, they have a high complication rate, including inflammation and further reduction in visual acuity.
Referring to FIG. 1, after production by the ciliary body, aqueous humor leaves the eye by many routes. Some goes posteriorly through the vitreous body to the retina, while most circulates in the anterior segment of the eye, nourishing avascular structures such as the lens and cornea, before outflow by two main routes: canalicular or uveoscleral.
The canalicular, also referred to as the trabecular or conventional, route is the main mechanism of outflow, accounting for approximately 80% of aqueous egress from the normal eye. The route is from the anterior chamber angle (formed by the iris and cornea), through the trabecular meshwork, into Schlemm's canal. The latter is a 360° channel just peripheral to meshwork. It is connected to intrascleral outlet channels that take the aqueous through the sclera to reunite with the blood stream in the episcleral veins.
The uveoscleral route is less clear with regard to anatomy and physiologic significance, but probably accounts for 10–20% of aqueous outflow in the normal human eye. As with the canalicular route, the uveoscleral pathway begins in the anterior chamber angle. The aqueous is absorbed by portions of the peripheral iris, the ciliary body and probably the trabecular meshwork, from whence it passes posteriorly through the longitudinal muscle of the ciliary body to the suprachoroidal space (between the choroids and sclera). Aqueous in the suprachoroidal space may pass as far posteriorly as the optic nerve and leave the eye through a variety of emissaria around nerves and vessels in the sclera.
A majority of operations that have been devised to enhance the aqueous outflow as a means of treating glaucoma have focused on enhancing canalicular outflow. The ideal glaucoma operation would be to re-establish normal canalicular flow into Schlemm's canal. In some forms of glaucoma this is possible, such as the iridectomy (introduced in the 1850s) for pupillary block glaucoma and goniotomy and trabeculotomy (introduced in the mid-twentieth century) for congenital glaucoma. For the vast majority of glaucomas, however, the obstruction to outflow (and, hence, the elevated intraocular pressure) is in the trabecular meshwork, and the only effective surgical approach has been to bypass the normal canalicular pathway and create bulk outflow by one of two methods: filtration surgery and drainage implant devices.
Filtration surgery was introduced in the first decade of the twentieth century. The basic principle is the creation of a fistula through trabecular meshwork, Schlemm's canal and sclera. Aqueous flows through the fistula to create a pool beneath the elevated conjunction (called a bleb), through which it filters to wash away in the tear film. The basic operation, in a variety of modified forms, has now been the preferred glaucoma procedure for nearly 100 years, despite serious limitations.
Limitations of filtering surgery include failure due to fibrotic closure of the fistula. Of even greater concern are the complications associated with excessive outflow, which include an intraocular pressure that is too low (hypotony) and a conjunctival filtering bleb that becomes too thin, with leakage and the risk of infection (endophthalmitis).
Drainage implant surgery was developed primarily to overcome the problem of fistula closure, since a conduit passes from the anterior chamber angle, through the fistula, to a plate beneath the conjuctiva. However, these operations are also complicated by early hypotony and late failure due to obstruction of the conduit or excessive fibrosis over the plate. There is a need, therefore, for a device and method of using same that reliably channels aqueous into pathways without creating hypotony or a filtering bleb.
Although the uveoscleral pathway may only account for 10–20% of aqueous outflow in the normal state, there is evidence that it can be enhanced to accommodate a significantly greater percentage of outflow. For example, topical prostaglandins, which work nearly exclusively by increasing uveoscleral outflow, can lower the intraocular pressure by 30–50% in some patients. Even more compelling are the results of early surgical attempts to enhance uveoscleral outflow.
In the first decade of the twentieth century, paralleling the introduction of filtering surgery, an operation was devised to enhance uveoscleral outflow, called cyclodialysis. Referring to FIGS. 2A and 2B, the basic principle is separation of the ciliary body from the scleral spur, which provides a direct route for aqueous flow from the anterior chamber angle to the suprachoroidal space. Unlike filtering surgery, however, cyclodialysis enjoyed only limited acceptance in the twentieth century. Although it was commonly used during the first half of the century, serious limitations led to its virtual abandonment by mid-century. The limitations were two-fold. When so-called cyclodialysis cleft was patent, the operation often worked too well, with significant hypotony. In many patients, the cleft would close suddenly, with a profound rise in the intraocular pressure.
A variety of efforts have been made to prevent closure of the cleft by wedging flaps of ocular tissue or plastic devices into the space. To date, none of these techniques have proved success.